hide CFR

we already set these

pref("browser.newtabpage.activity-stream.asrouter.userprefs.cfr.addons", false);
pref("browser.newtabpage.activity-stream.asrouter.userprefs.cfr.features", false);

but left the UI hanging out there (about:preferences > General > Browsing)

cc @henry @donuts

edit: we already do this for MB .. so we should just move that code to base browser